The present invention relates to an inspection apparatus for inspection using an image picked up by an optical camera, and a method for producing the image for inspection.
The system for handling radiation, for example, power plant has been demanded to ensure high safety, and required to execute sufficient inspection on a regular basis. Structures of the nuclear power reactor, for example, the reactor pressure vessel, the shroud in the vessel, the core support plate and the like are inspected as inspection objects. The inspection for objects other than those of the nuclear power reactor, for example, fuel assembly has been conducted.
The inspection method conducted by visually checking the surface condition of the inspection object using the optical camera has been employed as one of the inspection methods. In the visual checking, the camera is brought to close to the object, and the picked up image is shown on the display provided in a location with less radiation apart from the object so that the inspector performs the visual checking. The picked-up images are recorded so as to be confirmed later. The camera is provided with a remote-control operation function using a drive unit for mobility in the image pick-up area, or structured to allow the inspector to manually operate the camera from the place apart from the image pick-up area. Color or gray scale video information is obtained from the camera.
The inspection is conducted in the environment with high radiation intensity, for example, gamma ray, and noise is likely to be superimposed on the image picked up by the camera under the radiation influence, thus deteriorating visibility. This may cause the problem which hinders establishment of high reliability for inspecting soundness of the object. For this, the structure provided with radiation shield has been disclosed in Japanese Unexamined Patent Publication No. 9-311193 for reducing the influence of radiation on the camera.
The radiation may damage electronic circuit in the camera, and its functions as well. Especially, the recent miniaturized semiconductors tend to be susceptible to the damage. Once it is damaged, the camera element with high resolution, the one with wide dynamic range, and integrated circuit required for transmitting a large amount of image signals at high speeds hardly work. If durability against radiation is prioritized, the camera which employs few electronic circuits needs to be used as the one with low resolution and narrow dynamic range.
Meanwhile, Japanese Unexamined Patent Publication No. 10-221481 discloses the compact inspection device, and inspection device capable of traveling underwater aiming at the inspection in the narrow portion and easy operation.
The method is considered as applicable for reducing radiation noise by subjecting the obtained image to the image processing. Use of smoothing filter and median filter has been known as a general denoising method.
During the normal inspection, the lighting device is brought to be close to the object together with the camera for illumination. If the inspection object has a three-dimensionally complicated structure, the region which allows placement of the camera or the range which allows the illumination to reach are limited, making the illumination partially insufficient or excessive.
So another problem arises that it is difficult to pick up the video image by the camera for inspection under the appropriate illumination.
Japanese Unexamined Patent Publication NO. 2009-271096 discloses the method for executing contrast correction by obtaining correction formula based on brightness in the dark field and the brightness in bright field of the digital camera so as to improve visibility in reference to brightness of the image.
Japanese Unexamined Patent Publication No. 2009-65350 discloses the method for synthesizing a plurality of images each picked up by varying the exposure condition into the image in the digital camera field.
It is difficult for the method as proposed in Japanese Unexamined Patent Publication No. 9-311193 to reduce size and weight of the inspection device because of its radiation shield. In order to reduce the gamma-ray dose to 10%, the thickness of the apparatus needs to be 4 cm or larger while using lead which has been widely used as the gamma-ray shielding material.
Meanwhile, provision of the radiation shield for the device as disclosed in Japanese Unexamined Patent Publication No. 10-221481 is not practical from the aspect of size and weight.
General denoising process using the smoothing filter and median filter may cause problems as below. It is difficult for the smoothing filter and the median filter to appropriately suppress only the radiation noise while storing the component (signal component) except the radiation noise. The smoothing filter tends to deteriorate the high-frequency component of the signal to provide blurred images. The median filter provides substantially quality images when the noise amount is low, but may have its performance deteriorated when the noise amount is increased. When using the space filter with high accuracy such as load median filter besides those described above, improvement of the SN ratio (ratio of amount of signal component to radiation noise amount) is limited.
There is no image processing method for completely removing only noise in any images, and accordingly, deterioration in the signal component and residual noise are unavoidable to a certain degree. As to what degree deterioration in the signal component or the residual noise is allowed may vary depending on the inspection object and inspection type. There exists no interface which allows easy designation of the desired image in reducing the noise through the image processing.
During the actual inspection, there may be often the case that the inspection in wide range is conducted while moving the camera. In such a case, a plurality of positions with different radiation doses have to be inspected, and accordingly, the radiation noise amount contained in the image may vary as the camera moves. Under the environment with a small noise amount, denoising may be conducted relatively easily. However, under the environment with a large noise amount, it is difficult to conduct denoising. Therefore, it is difficult to provide quality image regardless of noise amount.
The method disclosed in Japanese Unexamined Patent Publication No. 2009-271096 applies the same contrast correction over the entire image, which fails to greatly improve visibility of interest region locally.
The method disclosed in Japanese Unexamined Patent Publication No. 2009-65350 requires a plurality of images with varied exposure conditions. However, if the inspection object has a three-dimensionally complicated structure to ensure reliability of inspection, it is difficult to arbitrarily change the exposure condition.
Under the radiation environment, the structure with radiation shield may be considered for reducing the influence of radiation on the camera. Such structure allows the use of high-performance camera which is hardly damaged by the radiation. In this case, it is difficult to reduce size and weight of the inspection apparatus because of the radiation shield. For example, in order to reduce the gamma-ray dose to 10%, the thickness of the structure needs to be 4 cm or larger while using lead which has been widely used as the gamma-ray shielding material. Therefore, it is not practical for conducting the inspection in narrow portion in terms of size and weight.
In the case where inspection is conducted using the image of the inspection object, which has been picked up by the camera, the method for creating the image with resolution higher than the pixel resolution of the camera may be considered. This method is capable of intensifying the resolution, but fails to improve the contrast of the image having the contrast partially lowered owing to insufficient or excessive illumination. The method is not regarded as the solution for the deteriorated visibility from the aforementioned aspect.